1. Field of the Invention
The present invention relates to a hydraulic suspension system including a suspension base supporting wheels, and a hydraulic cylinder for displaceably suspending the suspension base from a vehicle frame.
2. Description of the Related Art
European Patent Application Publication EP 0761481A2 discloses a work vehicle suspension system including a front axle frame supporting right and left front wheels, a support bracket supported to a vehicle frame to be vertically pivotable about a right/left axis, and a traveling suspension provided between the vehicle frame and the support bracket. With this suspension system, a vertically elongate bracket (suspension base) supporting an axle is mounted to be pivotable about a right/left axis of of a pivot shaft, and a hydraulic cylinder is provided between a front end of this vertically elongate bracket and a vehicle body frame.
Further, Japanese patent Application No. 2007-189612 filed by the present applicant on Jul. 20, 2007 discloses a hydraulic suspension system wherein a pair of right and left front wheels mounted to right and left opposed ends of a front axle case are suspended via a hydraulic suspension from a vehicle body frame.
With this suspension system, because of its structure, when there occurs a change in the load (sprung load of the suspension) applied to the front portion of the vehicle body as a result of attachment/detachment of an implement or variation in the work load, there occurs a change in the spring constant of the suspension in proportion to that load change. On the other hand, if the flow amount of work oil flowing between the hydraulic cylinder and an accumulator is constant, the damping coefficient of the suspension is constant. Therefore, if there occurs increase in the front load of the vehicle body due to e.g. attachment of an implement or increase in the work load and there occurs increase in the spring constant of the suspension in proportion thereto, the damping ratio of the suspension becomes smaller relatively, which leads to corresponding decrease in the force for deadening or damping vehicle body vibration. As a result, there occurs deterioration in the riding comfort and/or in stability during vehicle run.